Electrodes are widely used in many devices that store electrical energy, including primary (non-rechargeable) battery cells, secondary (rechargeable) battery cells, fuel cells, and capacitors. Important characteristics of electrical energy storage devices include energy density, power density, maximum charging rate, internal leakage current, equivalent series resistance (ESR), and durability, i.e., the ability to withstand multiple charge-discharge cycles. For a number of reasons, double layer capacitors, also known as supercapacitors and ultracapacitors, are gaining popularity in many energy storage applications. The reasons include availability of double layer capacitors with high power densities (in both charge and discharge modes), and with energy densities approaching those of conventional rechargeable cells.
Double layer capacitors use electrodes immersed in an electrolyte (an electrolytic solution) as their energy storage element. Typically, a porous separator immersed in and impregnated with the electrolyte ensures that the electrodes do not come in contact with each other, preventing electronic current flow directly between the electrodes. At the same time, the porous separator allows ionic currents to flow between the electrodes in both directions. As discussed below, double layers of charges are formed at the interfaces between the solid electrodes and the electrolyte. Double layer capacitors owe their descriptive name to these layers.
When electric potential is applied between a pair of electrodes of a double layer capacitor, ions that exist within the electrolyte are attracted to the surfaces of the oppositely-charged electrodes, and migrate towards the electrodes. A layer of oppositely-charged ions is thus created and maintained near each electrode surface. Electrical energy is stored in the charge separation layers between these ionic layers and the charge layers of the corresponding electrode surfaces. In fact, the charge separation layers behave essentially as electrostatic capacitors. Electrostatic energy can also be stored in the double layer capacitors through orientation and alignment of molecules of the electrolytic solution under influence of the electric field induced by the potential.
In comparison to conventional capacitors, double layer capacitors have high capacitance in relation to their volume and weight. There are two main reasons for these volumetric and weight efficiencies. First, the charge separation layers are very narrow. Their widths are typically on the order of nanometers. Second, the electrodes can be made from a porous material, having very large effective surface area per unit volume. Because capacitance is directly proportional to the electrode area and inversely proportional to the widths of the charge separation layers, the combined effects of the large effective surface area and narrow charge separation layers result in capacitance that is very high in comparison to that of conventional capacitors of similar size and weight. High capacitance of double layer capacitors allows the capacitors to receive, store, and release large amounts of electrical energy.
Electrical energy storage capability of a capacitor is determined using a well-known formula, to wit:
                    E        =                                            C              *                              V                2                                      2                    .                                    (        1        )            In this formula, E represents the stored energy, C stands for the capacitance, and V is the voltage of the charged capacitor. Thus, the maximum energy (Em) that can be stored in a capacitor is given by the following expression:
                                          E            m                    =                                    C              *                              V                r                2                                      2                          ,                            (        2        )            where Vr stands for the rated voltage of the capacitor. It follows that a capacitor's energy storage capability depends on both (1) its capacitance, and (2) its rated voltage. Increasing these two parameters is therefore important to capacitor performance. Indeed, because the total energy storage capacity varies linearly with capacitance and as a second order of the voltage rating, increasing the voltage rating is the more important of the two objectives for improving capacitors.
Voltage ratings of double layer capacitors are generally limited by chemical reactions (reduction, oxidation) and breakdown that take place within the electrolytic solutions in presence of electric field induced between capacitor electrodes. Electrolytic solutions currently used in double layer capacitors are of two kinds. The first kind of electrolytic solutions includes organic solutions, such as propylene carbonate. Long lifetime prior art double layer capacitors made with organic electrolytes can boast voltage ratings approaching 2.5 volts.
Double layer capacitors may also be made with aqueous electrolytic solutions, for example, potassium hydroxide and sulfuric acid solutions. Double layer capacitor cells manufactured using aqueous electrolytes and activated carbon are typically rated at or below 1.2 volts in order to achieve a commercially acceptable number of charge-discharge cycles. Even small increases above 1.2 volts tend to reduce substantially the number of charge-discharge cycles that the capacitors can withstand without significant deterioration in performance.
The 2.5 volt rating is considerably below voltage rating theoretically achievable in organic electrolyte-based double layer capacitors. According to some calculations, double layer capacitors made with an organic electrolyte and activated carbon should perform reliably at voltages ranging to about 3.2–3.5 volts. Achieving this range, however, has been an elusive goal because of early decomposition and breakdown of the electrolyte. The problem results, at least in part, from presence of impurities within the activated carbon and within the electrolyte. Water is one of these impurities.
Trace amounts of water and other impurities can be found in most electrolytes, and they may affect capacitor reliability, durability, and voltage rating. Highly purified electrolytes, however, are commercially available at reasonable cost.
The active material of the electrode—activated carbon or another porous material, for example—almost invariably has some impurities, including water. Water may be present in the raw carbon, and it may be introduced or added during the electrode manufacturing process. In practice, purifying activated carbon has proven to be a much more difficult task than purifying electrolyte. Water molecules can attach to the carbon in several ways, including by means of VanderWaal's forces responsible for physical bonding, and chemical (covalent and hydrogen) bonding forces.
Whatever the nature of the bond between a water molecule and activated carbon, a high energy “excited site” is formed around it. Electrolyte interacts with the water molecules and decomposes more readily near such sites than elsewhere in the capacitor. The trapped water functions deleteriously at the capacitor's working potential, so that the maximum application voltage is affected by the water devolution voltage. This is believed to be a major contributing cause to the lower actual-versus-theoretical breakdown voltage of double layer capacitors.
It would be desirable to increase actual breakdown voltage of double layer capacitors. It would also be desirable to increase reliability and durability of double layer capacitors, as measured by the number of charge-discharge cycles that a capacitor can withstand without significant deterioration in its operating characteristics. Because capacitor breakdown voltage and durability are both compromised by interaction between electrolyte and water molecules trapped in the activated carbon, it would be desirable to reduce the interactions or eliminate the interactions altogether. More generally, it would be desirable to provide a method for preventing impurities attached to porous materials from interacting with surrounding gas or liquid in which the porous material is immersed.